About Encryption

About encryption and encryption methods

WinZip's® encryption facility gives you a way to protect sensitive documents contained in your archives from unauthorized viewing. The contents of the files that you want to protect are encrypted by WinZip based on a password that you specify. In order for WinZip to later extract the original contents of the encrypted files, the correct password must again be supplied.

This section gives a general overview of WinZip's encryption facilities. Once you are familiar with this information, you can read about the specifics of using encryption.

WinZip provides two encryption methods for Zip files (.zip or .zipx):

AES encryption: AES is the Advanced Encryption Standard, which is the result of a three-year competition sponsored by the U.S. Government's National Institute of Standards (NIST). This encryption method, also known as Rijndael, has been adopted by NIST as a Federal Information Processing Standard.

WinZip supports AES encryption in two different strengths: 128-bit AES and 256-bit AES. These numbers refer to the size of the encryption keys that are used to encrypt the data. 256-bit AES is stronger than 128-bit AES, but both of them can provide significantly greater security than the standard Zip 2.0 method described below. An advantage of 128-bit AES is that it is slightly faster than 256-bit AES, that is, it takes less time to encrypt or decrypt a file.

The security of your data depends not only on the strength of the encryption method but also on the strength of your password, including factors such as length and composition of the password and the measures you take to ensure that your password is not disclosed to unauthorized third parties. Please read about encryption passwords and about password policy standards defined in the WinZip configuration.

Note that the Zip file format extension used by WinZip to store AES-encrypted files is supported by WinZip 9.0 or later and is supported by some other Zip file utilities. Because the technical specification for WinZip's AES format extension is available on the WinZip web site, we anticipate that other Zip file utilities will add support for this format extension.

Standard Zip 2.0 encryption: this older encryption technique provides a measure of protection against casual users who do not have the password and are trying to determine the contents of the files. However, the Zip 2.0 encryption format is known to be relatively weak, and cannot be expected to provide protection from individuals with access to specialized password recovery tools.

You should not rely on Zip 2.0 encryption to provide strong security for your data. If you have important security requirements for your data, you should instead consider using WinZip's AES encryption, described above.

The only advantage of Zip 2.0 encryption over the more secure AES encryption is that it is supported by most Zip file utilities, including earlier versions of WinZip. Files that you encrypt using this technique can be extracted by anyone who knows the correct password and has access to almost any Zip file utility. Additionally, Zip 2.0 encryption is supported by WinZip Self-Extractor 2.2 or later and by WinZip Self-Extractor Personal Edition (included with WinZip); the AES encryption method described above is only supported by WinZip Self-Extractor 3.0 or later.

Limitations to be aware of

WinZip's AES encryption facility represents a significant advance on the previous Zip 2.0 encryption, and it can help meet the need that many WinZip users have for preventing their confidential information from being viewed by unauthorized individuals. There are, however, some limitations that you should be aware of:

Encryption applies only to the contents of files stored within a Zip file. Information about an encrypted file, such as its name, date, size, attributes, CRC, and compression ratio, is stored in unencrypted form in the Zip file's directory and can be viewed, without a password, by anyone who has access to the Zip file.

WinZip's encryption method is not the same thing as an authentication method for the Zip file. WinZip encryption is intended to prevent someone who doesn't know the correct password from finding out the contents of your encrypted data. The password is not needed for actions that do not involve decryption of the encrypted contents of data stored within a Zip file. In particular, encrypted files can be deleted from a Zip file, or can be renamed within a Zip file, and new, unencrypted, files can be added to a Zip file, without a password.

WinZip uses password-based encryption, and even a strong encryption algorithm like AES is of little or no benefit if the passwords you use are weak, or you do not keep track of them in a secure manner.

We recommend that if you are going to be using the same password to encrypt very large numbers of files with WinZip's AES encryption (that is, files totaling in the millions, for example 2000 Zip files, each containing 1000 encrypted files) you use 256-bit AES keys rather than 128-bit AES-keys. The technical reasons for this are discussed below.

Notes on encryption safety

Encryption provides a measure of safety for your sensitive documents, but even encrypted documents can be compromised (regardless of whether they were encrypted by WinZip or by other encryption software). Here are some of the ways this can occur. This is by no means an exhaustive list of potential risks; it is intended only to give you an idea of some of the safety issues involved with sensitive documents.

If a keystroke monitor or other malicious code (such as a virus) is running on your computer, your password may be recorded when you type it. Be sure to check frequently for viruses and follow other recommended computer safety procedures.

If you extract an encrypted file and then delete the extracted file, it may be possible for someone to later "undelete" the extracted file using file recovery software or the Recycle Bin.

When you open or view a file from an archive (e.g., by double clicking it), WinZip must extract the file to a temporary location so that the associated program can open it. Normally, beginning with WinZip 14.0, when an encrypted file is temporarily extracted for these actions, WinZip will automatically wipe the temporary file prior to deleting it. However, if you subsequently close WinZip without first closing the program that is using the file, WinZip may not be able to wipe and delete the temporary copy of the file, thereby leaving it on disk in unencrypted form. The associated program may also make one or more backup copies of the decrypted file, and WinZip will not be able to wipe or delete those copies. If a file is unable to be wiped, it may be possible for someone to later recover deleted the file using file recovery software or the Recycle Bin.

When you "move" files to a Zip file by choosing the Move action in the Add dialog, WinZip moves the files into the Zip file by compressing them and then deleting the original files from the disk. WinZip will not wipe the original, unencrypted file prior to deleting it. It may be possible to recover the original, unencrypted files from the disk.

After adding or extracting encrypted files, some or all of the unencrypted file contents or of your password data may remain in your computer's memory or the page swap files on disk. A malicious user may be able to retrieve this unencrypted information.

WinZip does not encrypt Zip file comments or, as described above, information about encrypted files such as their names, dates, etc. Any user with access to the Zip file can view this information without a password.

You may be able to eliminate some of these exposures using specialized software such as virus scanners, disk erasers, etc.

Technical information on AES key generation

When you use AES encryption with WinZip, the passwords that you enter are converted into keys of the appropriate length (128 bits or 256 bits, depending on the AES key length that you specify). This is done through the PBKDF2 algorithm defined in RFC 2898 (also available as Public Key Cryptography Standard #5) with an iteration count of 1000. WinZip uses 8-byte salt values with 128-bit AES encryption and 16-byte salt values with 256-bit encryption.

One purpose for the "salt" values used with WinZip's AES encryption is to yield different encryption keys for each file, even if multiple files are encrypted with the same password. With the 8-byte salt values used with WinZip's 128-bit encryption it is likely that, if approximately 4 billion files are encrypted with the same password, two of the files will be encrypted with the same key. Someone who obtained copies of two files encrypted with the same key could learn information about their contents, so it is advisable to stay well below this limit. This is why we recommend that if you are going to be using the same password to encrypt very large numbers of files with WinZip's AES encryption (that is, files totalling in the millions, for example 2000 Zip files, each containing 1000 encrypted files), you use 256-bit AES keys, which use 16-byte salt values, rather than 128-bit AES-keys, with their 8-byte salt values.

As part of the process outlined in RFC 2898 a pseudorandom function must be called; WinZip uses the HMAC-SHA-1 function for this purpose, since it is a well-respected algorithm that has been in wide use for this purpose for several years. The PBKDF2 function repeatedly calls HMAC-SHA-1, which produces a 160-bit hash value as a result, mixing the outputs in a fairly complicated way, eventually yielding a 128- or 256-bit encryption key as a result.

Note that, if you are using 256-bit AES encryption, the fact that HMAC-SHA-1 produces a 160-bit result means that regardless of the password that you specify, the search space for the encryption key is unlikely to reach the theoretical 256-bit maximum, and cannot be guaranteed to exceed 160 bits. This is discussed in section B.1.1 of the RFC 2898 document.

Information for software developers

Zip file utility developers who wish to provide WinZip-compatible AES encryption support in their own products can find complete technical information on the WinZip web site.